Different methods allow technology teachers to approach the conception or the analysis of technical system. The origin of some methods is the functional analysis used in the industry. In France, the teacher in junior secondary schools uses them as classroom’s practices. With them the pupils can learn how the technical system works. But what do they really learn? Do we want them to apply a method or to understand the mechanism of a system? In this paper we study in which way the teachers interact with these instruments in the classroom. The study of the teacher’s practices is realized with a survey.